Illumination systems are used in many different applications, including projection display systems, backlights for liquid crystal displays and the like. Projection systems often use one or more white light sources, such as high pressure mercury lamps. The white light beam is usually split into three primary colors, red, green and blue, and is directed to respective image forming spatial light modulators to produce an image for each primary color. The resulting primary-color image beams are combined and projected onto a projection screen for viewing.
More recently, light emitting diodes (LEDs) have been considered as an alternative to white light sources. LEDs have the potential to provide the brightness and operational lifetime that would compete with conventional light sources. Current LEDs, however, especially green emitting LEDs, are relatively inefficient.
Conventional light sources are generally bulky, inefficient in emitting one or more primary colors, difficult to integrate, and tend to result in increased size and power consumption in optical systems that employ them.
Wavelength converted light emitting diodes (LEDs) are becoming increasingly important for illumination applications where there is a need for light of a color that is not normally generated by an LED, or where a single LED may be used in the production of light having a spectrum normally produced by a number of different LEDs together. One example of such an application is in the back-illumination of displays, such as liquid crystal display (LCD) computer monitors and televisions. In such applications there is a need for substantially white light to illuminate the LCD panel. One approach to generating white light with a single LED is to first generate blue light with the LED and then to convert some or all of the light to a different color. For example, where a blue-emitting LED is used as a source of white light, a portion of the blue light may be converted using a wavelength converter to yellow light. The resulting light, a combination of yellow and blue, appears white to the viewer. The color (white point) of the resulting light, however, may not be optimum for use in display devices, since the white light is the result of mixing only two different colors.
LEDs are also being used in image display systems, for example in television screens. In such applications an arrangement of individually addressable LEDs emitting red, green and blue light is used to illuminate one pixel. The relative luminance of each LED can be controlled so as to control the overall color perceived from the pixel.
In the newly-developing field of microprojection, a single LED imager device has been disclosed that includes individually addressable areas, pixels, that produce different colors, for example red, green and blue. The image is formed on the device by selectively illuminating the various pixels and projecting the image of the imager device.
Another approach to producing a color of light from an LED structure is the use of non-radiative coupling to transfer the energy from carriers in the active region of the LED to light emitting elements such as nanocrystals positioned on the surface of the LED. The nanocrystals subsequently emit light at a wavelength different from the wavelength of light had a photon been generated in the active region of the LED. In this technique, the LED does not emit pump light that is later converted to light at a different wavelength. Instead, the non-radiative dipole-dipole interaction between the nanocrystal and carriers in the active region transfers energy directly to the nanocrystal without a pump photon ever being formed. The strength of the dipole-dipole interaction, however, is strongly inversely dependent on the distance separating the nanocrystals from the active region, and so the upper doped layer of the LED, separating the active region from the nanocrystals, is necessarily very thin, around 10 nm or so.
Such a thin upper doped layer reduces the ability to spread current laterally from an electrode, and so the size and brightness uniformity of such devices is compromised. There remains a need to improve the performance of LED arrays that use light emitting, non-radiatively coupled nanocrystals for producing a uniform, large area emitter with differently colored pixels.